ATOLL RESEARCH BULLETIN NO* 272 HENDERSON ISLAND (SOUTHEASTERN POLYNESIA): SUMMARY OF CURRENT KNOWLEDGE F- R* FOSBERG, Me-H~ACHETAND D- Re STODDART SEPTEMBER 1983 Fig. 1. Henderson Island and surrounding area, South Pacific Ocean, based on the General Bathymetric Chart of the Ocean, 5th edition, sheet 5-11, 1980. Contents Page Introduction ................................... 1 Geography ...................................... 2 Archaeology .................................... 3 Discovery ...................................... 4 Later history .................................. 10 Scientific studies ............................. 14 Geology ........................................ 15 Vegetation and flora ........................... 16 Mammals ........................................18 Birds .......................................... 19 Reptiles ....................................... 22 Terrestrial arthropoda ......................... 22 Insecta ......................................24 Arachnida .................................... 25 Crustacea .................................... 25 Land Mollusca .................................. 26 Marine fauna ................................... 26 Fishes ....................................... 26 Mollusks .....................................27 Other Invertebrates .......................... 27 Scientific importance and Conservation ......... 27 Acknowledgements ............................... 28 Appendix 1. A revised list of the vascular plants of Henderson Island ................... 29 Appendix 2 . A revised list of the marine mollusks of Henderson Island by Harald A . Rehder ....................................... 34 Appendix 3 . Resolution of the Pacific Science Association adopted at the XV Pacific Science Congress ..................................... 40 References ..................................... 41 NE Point 240201s- Awahou F low trees Fig. 2. Henderson Island (Erorn the Admiralty chart). HENDERSON ISLAND (SOUTHEASTERN POLYNESIA): SUMMARY OF CURRENT KNOWLEDGE by F. R. Fosberg*, M.-H. Sachet* and D. R, Stoddart** INTRODUCTION The tropical seas are liberally sprinkled with coral islands. Many of them are associated with continents and large continental islands, in relatively shallow water, rising from continental shelves. Many, also, are in deep water - atolls, barrier reef islets round high islands, 'almost atollss, tops of drowned karsts, and a few moderately elevated atolls. These last are among the most fascinating of all, scientifically. There is no obvious reason to think that these are anything but ordinary atolls that have been tectonically elevated a few tens of metres above present sea level. Yet several features are frequently observed that are not evident on the sea-level atolls which are pres- ent in the coral seas in such large numbers. Terra rossa soils cover the limestone tops of some of them. Phosphate, earthy or indurated, or both, covers the tops and fills pits and crevasses of some,oc- casionally in enormous quantities, and is present at least in some quantities on most or all of them. Fantastically eroded, deeply and sharply pinnacled and pitted limestone surfaces are common. Endemic species of plants and animals, very rare on low atolls, are found in some numbers on almost all raised alolls of which we have even meagre knowledge, Whether or not these phenomena are related is at present not clear. Much further work on as many as possible of these islands is needed to elucidate this problem. The basic difficulty of the above statement is that, of the 20 or 30 such 'oceanic' islands or groups of islands, most have been greatly altered by long-established human occupancy, or phosphate min- ing, or both. Of the exceptions, the 'Rock Islands' of Palau and the Lau Group of Fiji are rugged karst, not comparable with raised atolls; Medinilla has been used as a bombing target; only Aldabra and Henderson remain reasonably unaltered. We are not here considering the many 'continental' limestone islands. --- * Botany Department, Smithsonian Institution **Geography Department, Cambridge University Aldabra and Henderson are topographically and physiographically very different, though both are probably raised atolls. Aldabra has been studied intensively during the past 17 years, is now well-known, and is legally protected. Henderson is relatively poorly known, and protected only by its remoteness and unsuitability for human habitation. During the past year the isolation and pristine condition of Henderson have been and are seriously threatened by a project of a wealthy American to build a house, landing facilities, and an airstrip on it. This is being opposed by scientific and conservation bodies, which are petitioning the British Government to deny permission to carry out this project. This circumstance has brought out and under- scored the paucity of reliable scientific information, except of a reconnaissance nature, about Henderson. This has caused the ARB editors to bring together a summary of what is known of the geography, history, ecology, flora and fauna of this island, with a bibliography of pertinent literature. This paper is, therefore, a factual record of the published information on Henderson: it sets down what has been discovered, at what time, and by whom, about the island. GEOGRAPHY Henderson is an elevated limestone island situated in 24"22'S and 128°20'~. Its nearest neighbours are Pitcairn, 200 km to the west- southwest; Oeno, 200 km to the west; and Ducie, 360 km to the east. It rises as an isolated conical mound from depths of ca 3.5 km, on a trend line which continues that of the Tuamotus and Gambiers eastward to Ducie, and is presumably a reef-capped volcano. The island has a greatest length of 9.6 km and greatest width of 5.1 km, based on the Admiralty chart; its area is 37 sq km. It is usually said to rise about 33 m (100 ft) above sea level. Early authors stressed the flatness of its summit, but those on the Mangarevan Expe- dition were impressed by a central depression which they interpreted as a former atoll-lagoon. The upper surface consists of an intricate dissected limestone, with pits and crevasses 3-7 m deep. The island is surrounded by steep cliPfs of bare limestone, with occasional pocket beaches, There is a fringing reef 200 m wide, at least on the north and northwest sides backed by a wide beach (st John and Philipson (1962)). No meteorological records are available, but the island lies in the Southeast Trades, and from its location probably has a mean annual rainfall of 1500 mm. The tidal range at springs is probably close to that of the Gambiers and eastern Tuamotus (1.0 m) (Admiralty Tide Tables 1983). Tidal measurements at Ducie by Rehder and Randall (1975) showed a regular semidiurnal tide. The top of the island, as well as any land at the bases of the cliffs, is densely vegetated by tangled scrub and scrub-forest, but the central part of the depression and the makatea are more sparsely covered. In places, where the forest is taller and the canopy more complete, it is possible to walk freely for short distances. The tallest trees are Pandanus tectorius Parkinson, which in places rises as an emergent above the general canopy. The crowns of such emergents are conspicu- ously pyramidal or conical. The dried fallen leaves of Pandanus cover the ground in many places. The scrub and much of the scrub-forest is in many areas so dense and tangled that walking through it is impossible without the constant and vigorous use of a machete, and exhausting and slow even with the use of one. Such scrub is as likely to be on dissected limestone as on solid soil, adding to the difficulty of traversing it. In some parts the difficulty is increased by the presence of Caesalpinia bonduc, a tangled vine beset with hooked prickles, which amply explain and justify a name used in Hawaii for this widely-distributed species, 'wait-a-bit'. Fresh water is almost completely absent. Slight dripping has been observed from the roofs of certain caves. A spring from a rock cleft below high tide level at the north end of the island has been reported (Naval Intelligence Division 1943, 92), but its degree of permanence is unknown. Presumably there is some development of a Ghyben-Herzberg lens within the island, but nothing is known of this, Biogeographically the island is very interesting because of its remoteness from obvious source areas. Comparison of its habitats with those of sea level and slightly-elevated atolls in the Tuamotus, in terms of effective salinity, proportions of different substrata (sand, dissected limestone, cliffs, level soil, etc.) is of great potential interest, but has yet to be considered except in a very preliminary manner. Study of the effects of geologically long continuous exposure above water, compared with the very short history of emergence of the sea-level atolls, could be rewarding. None of these lines of research can develop very far without much more information about the island than we have at present. Comparison with Aldabra would then be profit- able, ARCHAEOLOGY As noted above, Henderson has been, in historic times, uninhabited, with only brief visits by Pitcairn Islanders and occasional scientific parties. However, on one such visit, in 1971, Dr Yosi Sinoto, of the useum, discovered and made preliminary investigations on an archaeological site at the north end of the island. With his kind per- mission we quote his preliminary account of this discovery from a paper read in Nice in 1976: "Henderson Island, known also as Elizabeth